This invention relates to planar optical waveguides and, in particular, to a hybrid manufacturing method for such waveguides.
Planar optical waveguides are used as passive components in optical inter-connection systems. These waveguides are distinguished from cylindrical dielectric waveguides, e.g. optical fibers, in that they are substantially rectangular in cross-section. Existing methods for manufacturing these waveguides generally are expensive, require tight manufacturing controls, and result in waveguides with optical losses that are relatively high when compared to optical fibers.
Existing methods of producing planar optical waveguides involve the use of substrates having a first refractive index and having the preselected final dimensions of the planar optical waveguide to be formed. Materials having a second refractive index different from that of the substrate are applied to the substrate using various methods, including standard soot deposition techniques which are well-known in the art. (See, e.g. Keck et al. U.S. Pat. Nos. 3,806,223 and 3,934,061.) The preselected refractive index differential is achieved by using silica doped with one or more of the following: titanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconium oxide, aluminum oxide, lanthanum oxide, germanium oxide, or other suitable refractive index modifying dopant materials.
Optical circuitry within these planar waveguides is typically formed by a lithographic process similar to that used in the manufacture of semiconductor devices, as described in Izawa et al. U.S. Pat. No. 4,425,146. Another prior art method is described in Hudson U.S. Pat. No. 3,873,339 wherein a focused laser beam is used to fuse only that material which is to form part of the preselectcd optical circuitry, and the remaining unfused material is removed by cleaning or etching.
The use of lithographic techniques is wide-spread in the manufacture of semiconductor devices. These techniques are useful because detailed patterns--in the case of the present invention, optical circuit patterns--may be produced.
The lithographic process begins with a structure which contains the necessary materials to produce the desired electrical or optical circuit. This structure is coated with a photo-resistive material. The photo-resistive material is exposed to light through a mask which selectively exposes part of the photo-resistive material. The mask is the image of the desired circuit pattern. The exposed photo-resist is developed in a developing solution designed for the type of photo-resistive material used. The underlying structure is then etched using, for example, reactive ion etching to transfer the mask pattern to the underlying structure.
In the case of producing planar optical waveguides, a coating of alloy material, for example chrome, is typically applied to the underlying structure before the photo-resistive material is applied. This chrome layer is required because the photo-resistive material alone is not, in general, able to withstand the etching conditions necessary to etch the optical circuit into the underlying glass structure. The photo-resistive material is exposed and developed as above and the optical pattern is transferred to the intermediate chrome layer by using a chrome etching solution. Then the optical pattern is transferred to the underlying glass structure using, for example, reactive ion etching.
Each of these existing methods involves the application of very thin layers to form the core region of the waveguide. This core region guides the majority of the light through the waveguide. Small perturbations in the manufacturing process may result in inhomogeneous core structures with optical losses which are very high, particularly relative to the optical fibers which are attached to these planar optical waveguides. Therefore, tight control of the deposition process is required in existing methods to achieve the preselected thickness of the core region. This is particularly the case where the planar optical waveguide is manufactured for use in single-mode systems using fibers with core diameters of 10 .mu.m or less.
The problems inherent in existing methods of producing planar optical waveguides are:
1. optical losses are relatively high compared to those of optical fibers; PA1 2. expensive manufacturing controls are required to keep the optical losses to a minimum; and PA1 3. design and geometries are limited.
It is an object of this invention to produce planar optical waveguides with lower optical losses than those produced by existing methods. A second object is to combine the improved optical performance with the ability to produce mass quantities through the use of lithographic techniques, thereby making the inventive process more cost effective than existing methods. A third object is to provide planar optical waveguides with core layers of various shapes and with various refractive index profiles allowing planar optical waveguides to be used in a wide variety of applications.